The present invention relates to a wiring board having a blind through hole, and a method of manufacturing such a wiring board.
In a multilayer wiring board, if some conductor layers are connected with one another by a through hole passing through all layers, a substrate area in the other layers not required to be connected is also occupied by the through hole to cause a disadvantage from the viewpoint of high-density wiring. Accordingly, in designing a high-density multilayer wiring board, a so-called blind hole has been considered to be adopted. That is, a through hole is arranged between only the layers required to be connected, so as to increase a wiring density of the through hole and the pattern.
In general, formation of a through hole such as a via hole (i.e., a hole to be formed for the purpose of electrical connection only rather than insertion of parts) through an insulating substrate is carried out by drilling. However, if the blind hole as mentioned above is intended to be formed by drilling, the following problems are generated.
First, a tip of a drill to be used in drilling of a wiring board of this kind is mostly sharp, and it is difficult to control a depth of drilling. Therefore, it is difficult to precisely stop the drill at a certain position in the multilayer wiring board. Particularly in the case that a thickness of each insulating substrate is small, the adaptation of drilling to the board is difficult. It is also considered to use a drill having a flat tip. However, if a diameter of the drill is small, the drill is broken. In contrast, a large diameter of the drill counteracts the need of high-density wiring.
In another way, it has been developed that a plurality of double-sided substrates preliminarily formed with straight through holes are prepared, and these substrates are laminated to one another.
However, this method requires a high accuracy of positioning in laminating the substrates to make the working very troublesome. Further, there is a possibility that the through holes will be closed by adhesive or the like in laminating the substrates. In some case, there is a risk that resin will ooze out from an outermost through hole portion to largely damage the reliability of the wiring board. Additionally, the number of plating steps is increased to complicate a manufacturing step and increase a manufacturing cost. Moreover, as a thickness of a conductor to be etched is large, it is difficult to form a fine pattern.
In any methods as mentioned above, when a drill diameter is set to about 0.2 mm, a long working time is required to sacrifice a manufacturing cost. In contrast, when the drill diameter is set to about 0.3 mm or more, a working speed is in a level of mass production. However, such an increased drill diameter largely hinders the pursuit of reduction in diameter of the through hole for the purpose of high-density wiring.
Furthermore, in the case of adopting the drilling, the electrical connection between the conductor layers is relied solely on the contact between small end surfaces of the conductor layers exposed to a wall surface of a drilled hole and a plating layer formed on the wall surface of the drilled hole by through hole plating. Accordingly, the wiring board to be obtained has a disadvantage from the viewpoint of a current capacity, and it is also defective in connection reliability.
As another method to be substituted for the drilling, it is partly investigated to form a blind hole by working a polyimide resin with chemicals or laser. However, in the case that the insulating substrate is formed of a composite material consisting of the polyimide resin and a filler such as glass cloth, glass filler or aramid fiber, the working of the insulating substrate becomes difficult. In the case that the insulating substrate is formed solely of the polyimide resin, an insulation property, a mechanical strength, etc. of the insulating substrate become limited.
As described above, if the blind hole is intended to be formed by the prior art methods, there arise many problems in workability, reliability, etc., and there is a limitation in high-density wiring of the through hole and the pattern.